PTC (positive temperature coefficient) devices have come into wide use as circuit protection devices for a variety of electrical apparatuses or electronic apparatuses. Such PTC devices have characteristics in that the resistance thereof changes with a temperature, and rapidly changes (or increases) particularly at a specified threshold temperature, i.e., what is called a trip temperature. The characteristics that resistance increases, preferably rapidly increases, with a rise in temperature are called PTC characteristics.
In use, PTC devices are built in the electric circuits of electric or electronic apparatuses. For example, in case where some troubles occur in an apparatus in use to permit excessive current to flow into the electric circuit to raise the temperature of the apparatus, the temperature of the PTC device consequently reaches a threshold temperature and has very high resistance (e.g., the resistance becomes 1×101 to 1×104 or more times higher). As a result, if such a PTC device is built in an electric circuit and is present on a power supply line, the PTC device cuts off current to thereby prevent a failure in the electric apparatus. In case where an electric circuit having a PTC device built therein functions as a protection circuit in an apparatus, the resistance of the PTC device increases if an ambient temperature abnormally rises, and then, the PTC device switches a transistor in the protection circuit for detecting a change in voltage, so as to prevent a failure in the apparatus. Such PTC devices have been well known, and various types of PTC devices have been provided. For example, a PTC device is built in a protection circuit for electric circuits of a secondary battery in a portable telephone. If excessive current flows into the secondary battery in the course of charge or discharge of the battery, the PTC device cuts off the current to protect the battery.
As a typical example of existing PTC devices, a PTC device which comprises a laminar polymer PTC element produced from a polymer material having a conductive filler dispersed therein is known (see, e.g., Japanese Patent Kohyo Publication No. 10-501374 (1998, pages 7-15). The laminar polymer PTC element is produced by extrusion-molding a high density polyethylene which contains a conductive filler such as carbon black in a dispersed state. A PTC device is produced by disposing appropriate electrodes on both main surfaces of the polymer PTC element. Metal foil electrodes are used for such electrodes. The metal foil electrodes are bonded to the laminar polymer PTC element, for example, by thermocompression bonding.
The PTC device is built in a predetermined electric or electronic circuit by electrically connecting the metal foil electrodes thereof to metal lead elements. This electrical connection is generally done by caulking or soldering two elements, that is, the metal foil electrodes and the metal lead elements. In case of caulking, one of the elements has an opening, and the other element has a portion which has a complementary shape with a larger size relative to the opening, so that the portion of the other element is forced into the opening of the above one element so as to combine the two elements. Caulking, however, has a problem in that mechanical strength excessively acts on both of the two elements, resulting in possible damage in the PTC device.
In case of soldering, a soldering material is interposed and fused between two elements, however, the soldering material is required to be heated at high temperature for fusing. Recently, lead-free soldering has been proposed because a social problem arises because of lead contained in soldering materials. Generally, the lead-free soldering is carried out at higher temperatures than the conventional soldering, because of the higher fusing point. On the other hand, the metal foil electrodes of the PTC device are very thin and immediately transmit the heat for the soldering to the polymer PTC element, so that the polymer PTC element locally has high temperature to soften or melt. As a result, the dispersibility of the filler in the polymer locally becomes heterogeneous to change the PTC characteristics at such portions. Thus, there is a problem in that such change in PTC characteristics may give a serious influence on the performance of the PTC device as a whole. Therefore, a PTC element to be subjected to soldering is required to have a wide range of heat resistance in consideration of such an influence, and a PTC device comprising such a PTC element is demanded. Particularly in case of employing the lead-free soldering, a polymer PTC element having a far wider range of heat resistance is required.